1. Field of the Invention
The present invention relates to an image forming optical system and an electronic image pickup apparatus using the same.
2. Description of the Related Art
In recent years, with an increase in the number of pixels of an image pickup element and the progress of a digital image processing technology, a digital camera has substituted a silver salt 35 mm film camera. Moreover, since the number of pixels of a small-size liquid crystal panel which is used as a finder has increased, even an interchangeable lens camera is undergoing transition from a so-called single-lens reflex camera to a camera of a new concept in which, a quick-return mirror has been eliminated.
Furthermore, since a capacity of a recording medium such as a flash memory and a bit rate of an image processing system has increased in units of digits, video recording of a high image quality is becoming possible. However, for small-sizing of a camera system while mounting a video function of an improved performance, it is necessary to make a focusing-drive portion light.
Lately, for making the focusing-drive portion in a zooming optical system as light as possible, a lens group of a rear portion of an optical system having a small diameter and a fewer number of lens components has been selected. Particularly, the lens group of the rear portion of the optical system includes only one lens component. However, in a camera in which, an image forming performance beyond a certain degree is sought from infinity to an object-point at a close distance, correction of a chromatic aberration, a coma aberration, and a meridional curvature of field is insufficient.
Therefore, for forming a focusing lens group by a plurality of lens components while maintaining a light weight of the focusing lens group, for a lens group more on an image side than an aperture stop, using a lens group which includes one component in which, a thin resin lens is cemented to a base-material lens as a focusing lens group as proposed in Japanese Patent Application Laid-open Publication Nos. 2007-108707, 2007-108715, 2008-191308, and 2008-191311, is to be taken into consideration.